Update lighttpd-auth.conf

Added 3rd expression variant.
Greps when trying to login with not existing Username.
System:
Linux version 2.6.32-5-amd64 (Debian 2.6.32-48squeeze1) (dannf@debian.org) (gcc version 4.3.5 (Debian 4.3.5-4) ) #1 SMP Mon Feb 25 00:26:11 UTC 2013
lighttpd/1.4.28 (ssl) (Mar 15 2013 19:01:10) - a light and fast webserver